The present invention generally relates to a copying apparatus and more particularly, to an electrophotographic copying apparatus with an improved exposure amount control for higher accuracy in the density control of the copy.
Conventionally, there has been proposed a copying apparatus provided with an exposure amount control arrangement adapted to detect a reflected light amount or transmitted light amount of an original document for automatically controlling the exposure amount with respect to the original document according to the detected light amount, thereby to achieve optimum density, i.e., dark and light portions corresponding to the original document in copying.
In connection with the above, for example, in Japanese Patent Laid-Open Application Tokkaisho Nos. 57-29060, 57-124373 and 57-124374, there are disclosed arrangements in which a photo-detecting element is disposed outside the center of a light path in a position after an image forming optical system so as to control the exposure amount according to output signals of the photo-detecting element. However, since the above photo-detecting system can detect only the amount of reflected light from a very limited part of the original document, it is impossible to obtain data correctly reflecting the overall state at the dark and light portions of the original document, and thus, an accurate exposure control may not be effected.
With respect to the above point, in Japanese Patent Laid-Open Application Tokkaisho No. 57-2134, there is proposed an exposure control arrangement in which a plurality of photo-detecting elements are disposed to correspond to the widthwise direction of an original document (or original document platform) at positions optically conjugated with respect to the image forming surface of a photosensitive member or photoreceptor, so as to control the exposure amount by generating the respective outputs of such photo-detecting elements. In the above known system, although a comparatively accurate exposure control may be effected since the degrees of dark portions and light portions in the widthwise direction of the original document are detected, it becomes necessary to cause a mirror or the like for directing light toward the photo-detecting elements, uniformity in the sensitivity is required for the respective photodetecting elements, with simultaneous complication of the control means. Another problem related to the above conventional system is such that, depending on the original document sizes, some of the photo-detecting elements receive reflected light not from the original document, but from an original document cover, and thus, erroneous data can be undesirably taken into account.
Furthermore, in Japanese Patent Laid-Open Application Tokkaisho No. 57-45564, there is disclosed an exposure amount control device which is so arranged that, by causing a photo-detecting sensor to scan by a sensor driving unit in a direction intersecting at right angles with an optical system moving direction so as to prepare a histogram corresponding to the density distribution at respective points in the widthwise direction of an original document, a characteristic pattern of said histogram is derived for controlling the exposure amount.
However, the known device as described above has such problems that, besides the fact that the driving unit for causing the photo-detecting sensor to scan in the direction normal to the optical system moving direction must be constructed on a large scale, density of a part other than the original document, for example, density of an original document cover taken into the histogram as in the arrangement described earlier, unless the scanning width is accurately controlled according to the original document.
As described so far, conventional image density control arrangements have disadvantages in that, since dark and light portions only at a limited part of the original document can be detected or dark and light portions on parts other than the original document can not be accurately detected, thus making it impossible to correctly control the darkness or lightness of copied images.